fix: empty signature added after every certificate saving (#30912)

[melvinsoft]

Cherrypicking from upstream

A new behaviour:

• Empty signature is still added when initially create a certificate;
• Empty signature isn't added when certificate has at least one signature.

Type of change

• Bug fix (fixes an issue)
• New feature (adds functionality)

Related issues

Checklists

Development

• Lint rules pass locally
• Application changes have been tested thoroughly
• Automated tests covering modified code pass

Security

• Security impact of change has been considered
• Code follows company security practices and guidelines

Code review

• Pull request has a descriptive title and context useful to a reviewer. Screenshots or screencasts are attached as necessary
• "Ready for review" label attached and reviewers assigned
• Changes have been reviewed by at least one other contributor
• Pull request linked to task tracker where applicable

[github-actions]

Checking git merge conflicts against https://github.com/edx/edx-platform.git

Comparing with	open-release/koa.master
Benchmark conflicts with main	111
Current conflicts	111
Summary	Good work! No added conflicts.

Comparing with	open-release/lilac.master
Benchmark conflicts with main	253
Current conflicts	253
Summary	Good work! No added conflicts.

Comparing with	open-release/maple.master
Benchmark conflicts with main	284
Current conflicts	284
Summary	Good work! No added conflicts.

Comparing with	open-release/nutmeg.master
Benchmark conflicts with main	292
Current conflicts	292
Summary	Good work! No added conflicts.

Comparing with	master
Benchmark conflicts with main	290
Current conflicts	290
Summary	Good work! No added conflicts.

[coveralls]

Pull Request Test Coverage Report for Build 3153221853

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.0009%) to 49.035%

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Totals
Change from base Build 3141507176:	0.0009%
Covered Lines:	110926
Relevant Lines:	226218

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💛 - Coveralls

[OmarIthawi]

great!! thanks